1. Field of the Invention
The present invention relates to an electrical discharge tube in which electrical discharges are repeatedly induced between an electrical discharge face at a forward end of an upper discharge electrode and an electrical discharge face at a forward end of a lower discharge electrode which are opposed to each other at the center in an airtight cylinder.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 10-335042 discloses an electrical discharge tube used for a ballast circuit to ignite an HID (high intensity discharge) lamp of a vehicle and also used for an igniter circuit to ignite a back side lamp of a liquid crystal projector.
As shown in FIGS. 37 and 38, this electrical discharge lamp is provided, in the traverse direction at the center of the inside wall of the airtight cylinder 10, with a plurality of main electrical discharge trigger wires 80 which are arranged at predetermined intervals, while the main electrical discharge trigger wires 80 rise in the vertical direction in parallel with the axis of the airtight cylinder 10. On the upper inside wall of the airtight cylinder 10 between the main electrical discharge trigger wires 80, there are sub-electrical discharge trigger wires 90 which rise in the vertical direction in parallel with the axis of the airtight cylinder 10, and upper ends of these sub-electrical discharge trigger wires 90 are serially connected with the metalized face 40 formed on the upper end face of the airtight cylinder 10. In the same manner, on the lower inside wall of the airtight cylinder 10 between the main electrical discharge trigger wires 80, there are sub-electrical discharge trigger wires 90 which rise in the vertical direction in parallel with the axis of the airtight cylinder 10, and the lower ends of these sub-electrical discharge trigger wires 90 are serially connected with the metalized face 40 formed on the lower end face of the airtight cylinder 10.
In this electrical discharge tube, it is possible to prevent the electrical insulation between the main discharge trigger wires 80 and the sub-discharge trigger wires 90, which are arranged on the inner wall of the airtight cylinder 10, from being deteriorated by sputtering such as carbon particles which are created in the process of discharging from the electrical discharging face 23 at the forward end of the upper discharge electrode, the electrical discharging face 25 at the forward end of the lower discharge electrode, the main electrical discharge trigger wires 80 and the sub-electrical discharge trigger wires 90 and which adhere to the center of the inner wall of the airtight cylinder 10. Further, in this electrical discharge tube, it is possible to repeatedly and stably induce electrical discharges between the electrical discharging face 23 and the electrical discharging face 25 at a predetermined electrical potential over a long period of time.
In general, in an igniter circuit which uses the commercial power source as a power source and performs an electrical discharge synchronizingly with the frequency of that power source, an electrical discharge gap is arranged at the side of the secondary coil opposite to the primary coil of the transformer.
However, there are some ballast circuits or igniter circuits including electrical resistors, coils, or the similar parts mounted at a high density, which are uses for igniting the HID lamp or the like, as mentioned abode. In such a circuit, the electrical discharge tube constituting an electrical discharge gap is arranged close to the primary booster coil in the circuit, and the direction of the winding of the primary booster coil is substantially perpendicular to the direction of the main electrical discharge trigger wires 80 and the sub-electrical discharge trigger wires 90.
Therefore, the main electrical discharge trigger wires 80 and the sub-electrical discharge trigger wires 90 are affected by the magnetic field generated by the primary booster coil, and an electrical current is generated by the electromagnetic induction caused by the main electrical discharge trigger wires 80 and the sub-electrical discharge trigger wires 90. Being affected by the electrical current, the electrical potential of electrical discharges repeatedly induced between the electrical discharge face 23 and the electrical discharge face 25 cannot be stabilized, that is, the electrical potential of electrical discharges fluctuate and, further, the electrical discharge starting voltage initially generated between the electrical discharge face 23 and the electrical discharge face 25 is raised.
The above ballast circuit used for igniting the HID lamp of a vehicle is embedded and fixed in resin such as urethane resin or epoxy resin so that the circuit can be protected from impact and vibration, and the electrical discharge tube composing the ballast circuit is surrounded by the dielectric resin.
Therefore, the electrical discharge tube is affected by the dielectric resin. Accordingly, it is impossible to effectively converge electrons of the corona discharge upon the sub-electrical discharge trigger wires 90 of the electrical discharge tube. Further, the electrical discharge starting voltage initially generated between the electrical discharge face 23 and the electrical discharge face 25 is raised.